1. Field of the Invention
This invention relates to a fuel ignition system of the pilot ignition type employing a thermal heat sensor which is responsive to main burner heat and which effects the shutoff of the fuel supply to both pilot and main burners following flame-out under certain failure conditions.
2. Description of the Prior Art
Fuel ignition systems of the pilot ignition type, exemplified, for example, by the system shown in my U.S. Pat. No. 3,902,839, which was issued on Sept. 2, 1975, include a pilot valve which is operated in response to the closure of thermostatically controlled contacts to supply fuel to a pilot outlet for ignition by a suitable igniter to establish a pilot flame. A pilot flame sensing circuit detects the pilot flame and effects the energization of a main valve which supplies fuel to a main burner apparatus for ignition by the pilot flame. Typically, the operation of the main valve is controlled by a relay of the flame sensing circuit which has normally open contacts connected in the energizing path for the main valve.
In the event the pilot flame fails to be established within a predetermined time interval, the flame sensing circuit maintains the relay deenergized to prevent the main valve from operating. Also, in the event of a flame-out condition following a successful start-up, the flame sensing circuit effects deenergization of the relay to deenergize the main valve until the pilot flame is established.
However, should the relay contacts which control the energization of the main valve become welded together following a successful ignition cycle, then, in the event of a flame out condition, the deenergization of the relay of the flame sensing circuit will be ineffective to provide deenergization of the main valve. For such simultaneous failure conditions, both the pilot valve and the main valve will remain energized, and fuel will emanate from the pilot outlet and the main burner, an undesirable condition.
Therefore, it would be desirable to have a control arrangement for use in fuel ignition systems for effecting the deenergization of the pilot valve and the main valve under simultaneous failure conditions of the type noted above.